Here's one simple example. A seed compost has to be fine. But some multi-purpose composts contain hard lumps of peat.
So be aware that 'Multi-purpose' really over stretches the capability of a compost. It’s more about marketing than good growing - a simple reply to the many needs gardeners now have for compost, from seeds to hanging baskets, from tomatoes to shrubs.
Peat Base For Compost: All multi-purpose composts are based on peat. There’s no doubt it has great value. Many gardeners believe peat is an ideal and irreplaceable base for making compost. But as peat harvesting destroys sensitive bog habitat along with rare plants and animals, commercial peat harvesting is not sustainable. The UK target is to make 90% of compost / soil conditioner sold by 2010, peat free. So vendors are now reducing the peat content.
Peat Alternatives: The alternatives to peat are usually based on composted bark, wood and fibre. They are still in development. One problem reported by gardeners occurs if the wood has not been fully composted. This can result in microbes removing nitrogen to balance the excess carbon. It leads to nitrogen shortage for plants and reduced growth. Some peat alternatives may lock up trace elements copper and molybdenum which can again result in nitrogen shortage. Thus composted materials can lead to variation in quality. But a good compost gardener can make suitable organic alternatives to peat.
Which Proprietary Compost: The Which? report 'Compost on Trial 2004' gave J Arthur Bower's New Horizon Multi-Purpose Compost as the best peat free compost; acceptable for seeds, good for growing on and sustaining large pots and for summer watering.
However, it's probably better to add ingredients to a base so as to adapt it for particular uses. For example plants grown on in J Arthur Bower's New Horizon Multi-Purpose may have scored even better if augmented by watering plants with seaweed extract or fish emulsion.
The Which report also gave Westland Multi-Purpose Compost with added John Innes (80% peat) and Gem Multi Purpose Compost (80% peat) and J. Athur Bower’s Multi-Purpose Compost (80% peat) as good for seeds, growing on, large pots and watering.
Composts with added John Innes: Composts with added John Innes are especially noteworthy. The added loam gives them better fertilizer and water holding capability; and when dry they are less susceptible to shrinking and more easily rewetted. (Once dry peat is difficult to re-wet.)
An organic compost garden: Most compost formulae from John Innes onward include inorganic fertilizers in the mix, like potassium sulphate or superphospate. But good organic gardening compost contains a healthy microbe population that effectively feeds the plant from slow release organic nutrients. This root microbial relationship is better for plant roots.
Consider top dressing with worm casts, mixing fine bone meal into potting mixes, or by using garden compost in your home-made organic compost mixes.
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